[T10] SPC5r20a - - Allocation length confusing.

Gerry Houlder gerry.houlder at seagate.com
Wed Feb 27 15:40:19 PST 2019

That wording was intended to cover the case where There are different size
commands that can do the same function. For example, MODE SENSE(6) has only
a one byte transfer length and MODE SENSE(10) has a two byte transfer
length. If a target has 300 bytes of mode data to transfer and the host
issues a MODE SENSE(6) command, the command should end with CHECK CONDITION
status with the allocation length error. This is supposed to clue the host
to use the MODE SENSE(10) command to return the data. If more than FFFFh
bytes of mode data was available, a MODE SENSE(10) command should truncate
the data (return as much as it can) and it is up to the host to send a
different MODE SENSE(10) command that will retrieve a different set of data
If the host didn't get the data it wanted.

On Wed, Feb 27, 2019 at 5:30 PM Kevin D Butt <kdbutt at us.ibm.com> wrote:

> The key is to pay attention to the words, "exceeds the maximum value that
> the ALLOCATION LENGTH field in combination with other fields in the CDB
> is capable of specifying". This is not talking about what value is in the
> allocation length field, but the maximum value that could be specified. If
> there is more data to transfer than what can be specified, then return the
> CHECK CONDITION. e.g., allocation length field is 4 bytes. Information
> available to be transferred is greater than 2^32 (4294967296), then Check
> Condition is returned.
> Kevin D. Butt
> SCSI Architect, Tape Firmware, Data Retention Infrastructure
> T10 Standards
> MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
> Tel: 520-799-5280
> Fax: 520-799-2723 (T/L:321)
> Email address: kdbutt at us.ibm.com
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> ===================================
> From:        <t10 at terabyteunlimited.com>
> To:        <t10 at t10.org>
> Date:        02/27/2019 16:07
> Subject:        [T10] SPC5r20a - - Allocation length confusing.
> Sent by:        t10-bounces at t10.org
> ------------------------------
> Hello,
> If you read the section in the 3rd paragraph it says to transfer bytes
> until ALLOCATION LENGTH field has been transferred (so truncation can occur
> and size needed report).  Yet, the last paragraph says if what you have to
> transfer is more bytes than ALLOCATION LENGTH then don’t transfer anything
> and report an error.   So which is it?  REPORT LUNS for example.
> * Allocation length*
> The ALLOCATION LENGTH field specifies the maximum number of bytes or
> blocks that an application client has allocated in the Data-In Buffer. The ALLOCATION
> LENGTH field specifies bytes unless a different requirement is stated in
> the command definition.
> An allocation length of zero specifies that no data shall be transferred.
> This condition shall not be considered an error.
> The device server shall terminate transfers to the Data-In Buffer when the
> number of bytes or blocks specified by the ALLOCATION LENGTH field have
> been transferred or when all available data have been transferred,
> whichever is less. The allocation length is used to limit the maximum
> amount of variable length data (e.g., mode data, log data, diagnostic data)
> returned to an application client. If the information being transferred to
> the Data-In Buffer includes fields containing counts of the number of bytes
> in some or all of the data (e.g., aPARAMETER DATA LENGTH field, a PAGE
> LENGTH field, a DESCRIPTOR LENGTH field, an AVAILABLE DATA field), then
> the contents of these fields shall not be altered to reflect the
> truncation, if any, that results from an insufficient ALLOCATION LENGTH value,
> unless the standard that describes the Data-In Buffer format states
> otherwise.
> If the amount of information that is available to be transferred exceeds
> the maximum value that the ALLOCATION LENGTH field in combination with
> other fields in the CDB is capable of specifying, then no data shall be
> transferred and the command shall be terminated with CHECK CONDITION
> status, with the sense key set to ILLEGAL REQUEST, and the additional sense
> code set to INVALID FIELD IN CDB.
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